At some point during summer 2010, pixiv split manga-mode images into two sizes: a "sample" version that is first displayed on the mode=manga pages that uses the old-style URL (e.g. "http://img61.pixiv.net/img/alkjsld/1498651_p0.jpg") and a new "big" size linked to (if you are logged into pixiv) by a small square icon above each page.
For images before this change, the image within has the exact same URL.* For later images, however, the links display images with a new "big" style URL (e.g. "http://img61.pixiv.net/img/alkjsld/1498651_big_p0.jpg"). The images might be exactly identical, barely different, or significantly different to their old style counterparts. Unlike pixiv thumbnails, there doesn't seem to be any hard rule on when and how much images are resized.
This tag applies to all posts whose MD5 hashes match the sample versions' and not also the original sizes'. Do not add this tag unless you can confirm a difference, or remove it unless the hashes match.
If the image is of acceptable quality, upload the "big" size, make it the parent of the sample version, and flag the sample for deletion.
See forum #51675 for more discussion on this development.
For the lower-resolution versions of non-manga-mode images, see pixiv thumbnail.
*If a manga-mode image from before the change undergoes a revision (indicated by a ?####### tacked on to the image source) after the change, a "big" size will be created, but pixiv won't display it where it should. This hidden size can still be uploaded, however, by manually adding "_big" to the URL.
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